The present invention pertains to quadrature mixers of the type in which an input signal is mixed with a quadrature local reference signal. The invention does not pertain to quadrature mixers of the type where the input signal itself is also quadrature. Quadrature mixers of the variety to which the present invention pertains, have many potential practical applications, including integrated circuits and modules for radio frequency receivers, and other wireless communications products. They are, for instance, employed in single side-band mixers and quadrature demodulators and modulators.
Single side-band mixers are used in cellular telephone technology for heterodyning an incoming radio frequency (RF) cellular telephone signal down to an intermediate frequency. The use of a single side-band mixer employing a quadrature mixer for this purpose performs two functions which otherwise would need to be performed by separate circuits. In particular, a single side-band mixer using a quadrature mixer not only performs heterodyning, but also can be used to image reject one side-band of a double side-band (DSB) input signal.
Because the frequency ranges dedicated to cellular telephone communications have rapidly become overloaded with communication traffic, both the United States and Europe have recently added new band ranges dedicated for cellular telephone communications. Particularly, the U.S. has added 1900 megahertz as a second cellular telephone communication band to the previously existing 800 MHz range. In Europe, where 900 megahertz is dedicated to cellular telephone communications, 1800 megahertz has been added as a second frequency for cellular telephone communications.
In view of these new bands for cellular telephone communications, there is a need for cellular telephones which can receive signals at two separate carrier frequencies, e.g., in the U.S., 800 megahertz and 1900 megahertz.
One way to provide such functionality in a cellular telephone is to provide two sets of receive circuit paths, one for each of the two possible carrier frequencies.
However, at the same time that these new band widths for cellular telephone communication are being opened, there is the ever-present drive to reduce the size of electronic devices, including cellular telephones. Accordingly, the solution of simply adding a second receiver path is undesirable since it significantly increases the circuitry in the telephone, and thus the telephone's size.
Therefore, it is desirable to reduce the size of the circuits needed in the telephone in order to perform the necessary functions. It also is desirable to provide a single circuit which can perform the same function at two different frequencies in order to reduce the overall amount of circuitry needed to accommodate both frequency ranges.